Air Traffic Control (ATC) is now supplementing voice communication with datalink communications between air traffic controllers and pilots. The datalink communication system is specified worldwide by standards from the International Civil Aviation Organization (ICAO). The datalink communication system uses a communication protocol suite called Aeronautical Telecommunication Network (ATN), which is based on the standard seven layer Open Systems Interconnection (OSI) protocols.
One layer of the ATN is the transport (TPS) layer, which contains a retry timer (T1) and logic to manage the retry timer. When the retry timer expires, a Transport Protocol Data Unit (TPDU), which contains a “message” or part of a “message” is transmitted again by the transport layer, the value of the retry timer is doubled, and the retry timer is started again. This continues until a TPDU acknowledging the transmitted TPDU is received or the maximum number of transmissions has occurred.
The airborne implementation of ATN is subject to brief interrupts in the datalink service. The datalink service is the actual wireless radio frequency (RF) communication between the aircraft and ground. The datalink service carries messages, such as TPDUs, from the higher layers of the ATN stack, which are designed to ignore brief datalink service interruptions and try to “coast” through the interruptions. This feature is called “hold mode.”
When ATN enters a hold mode, ATN continues to operate even though the datalink layer (DLL) connection to the ground has been briefly interrupted. The TPDU does not actually get transmitted to the ground by the datalink service during the hold mode. However, the retry timer logic continues to execute and the retry timer continues to expire, with the value of the retry timer doubling each time the retry timer expires. This is due to a dynamic retransmission timer recommended by the ATN industry specifications. The algorithm for the retransmission timer is defined in ICAO 9880.
Accordingly, the current implementation of the retry timer logic can result in message delays that are much longer than ATN unavailability due to brief interruptions in the datalink service.